Currently there are a number of glass families designed for high strength and high modulus that are at least capable of forming fibers. For example, R-Glass and S-Glass are examples of such glass families, both of which have higher strength than boron-containing E-Glass. R-Glass is considered a high strength, high modulus glasses that is formable into fibers and useful in aerospace composite applications. R-Glass is typically composed of silicon oxide, aluminum oxide, magnesium oxide, and calcium oxide.
S-Glass is typically composed of silicon oxide, aluminum oxide and magnesium oxide (ASTM International (D578-05)). S-glass fibers have a slightly higher mechanical strength, in part from its higher silica content and less calcium oxide content than R-Glass fibers. The chemical composition of the S-glass family also provides for high strength glass fiber useful in high strength applications, such as ballistic armor. Deutsches Institutflir Normung (DIN) classifies S-Glass as an aluminosilicate glass (e.g., mostly aluminum trioxide and silicon dioxide) and having MgO at about 10% by weight (DIN 1259-1) without added CaO.
One drawback to the R-Glass and S-Glass families is that they require higher melting and processing temperatures during fiber forming than that of E-Glass, which requires that R-Glass and S-Glass be generally manufactured by melting the constituents of the compositions in specialized melter such as a platinum lined melter, thus raising the production costs of forming R-Glass and S-Glass fibers compared to E-Glass fibers. In addition, higher processing temperatures reduce the life of the fiber bushings, which are expensive to replace. Ideally, a continuous fiberization process utilizing a refractory lined melter and low fiber bushing temperatures would be desirable in combination with a high-strength glass composition.